Angular momenta polarization via depopulation

For many years there existed the widespread opinion that in molecules we obtain an anisotropic distribution of angular momenta only in the excited state in the absorption process, whilst in the ground state isotropy is conserved [124]. This conjecture is based essentially on the assumption of the existence of a so-called weak excitation. At first sight it may appear, for instance, from Eq. (2.20) that, as is usual in non-linear spectroscopy, such an admission is correct if condition rp/r C 1 is valid. This may not be the case for molecular systems, since spontaneous emission does not 

The latter mechanism is excluded for homonuclear diatomic molecules (dimers) owing to the absence of a permanent dipole moment in these molecules. In the case of heteronuclear molecules, on the other hand, such transitions are well known and are widely employed, in particular for the determination of configurational and relaxational parameters by methods of infrared spectroscopy. 

The transit time is only the upper limit for the existence of light-produced optical anisotropy of angular momenta in the lower state. Under 

Let collisions AB-X between a certain molecule AB, in the electronic ground state a , on a selected rovibrational level v , J , and at given spherical angles 9, ip determining angular momentum Ja orientation, and a particle X, tend to restore the equilibrium state population and the isotropic distribution of momenta Ja in a bimolecular reaction  

It is important to produce a simultaneous description of collisional and fly-through relaxation. At first sight this appears to be a simple enough problem, namely, similarly to the excited state (2.42), one must introduce the total rate 7s additively  

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